COPENHAGEN—One thing is clear under this city's low, leaden skies: a ton of carbon dioxide emitted in India is the same as a ton of carbon dioxide emitted in the U.S. And while some notable contrarians are present here at the United Nations' climate summit, their presence is going largely unnoticed because representatives from most countries accept the basic physics of a molecule of carbon dioxide trapping heat in the atmosphere. But what is less clear is how the various negotiators from 193 countries are incorporating new developments in the scientific understanding of climate change and its progress, particularly as various draft texts of what an agreement might look like circulate through the crowded halls of the Bella Center.

"Our starting point is the environment," says Karl Falkenberg, director general of the environment at the European Commission. "What we're looking for here is an outcome that effectively deals with climate change and the consequences of climate change that we are seeing everywhere on the globe."

The general consensus seems to be that global emissions of greenhouse gases must be cut by 50 percent from 1990 levels by 2050 with developed countries and bodies such as those in the European Union (E.U.), Japan and the U.S. making larger cuts to allow developing countries, such as China and India to grow. "Different commitments we can accept but they need to be verified in a coherent manner," Falkenberg says. "Continued carbon-based economic growth in developing countries is contributing to poverty not alleviating it."

But under previous agreements, such as the original United Nations Framework Convention on Climate Change agreed upon in 1992, developed countries' emissions were supposed to peak in 2000 and thereafter decline. Instead, "emissions have risen since 2000," notes Indian ambassador Chandrashekhar Dasgupta. In fact, concentrations of carbon dioxide in the atmosphere rise by roughly 2 parts-per-million a year, having reached roughly 387 ppm to date. And some scientists, such as NASA climatologist James Hansen, argue that anything above 350 ppm is dangerous, given impacts such as melting Arctic sea ice that are happening much faster than scientists had anticipated.

"We need to have targets that are science-based," Falkenberg says. The E.U. has settled on targets of keeping warming from exceeding a 2 degree Celsius rise in average temperatures (average temperatures have already warmed 0.7 degree C), which roughly equates to CO2 concentrations in the atmosphere of 450 ppm.

Of course, many scientists feel that on present efforts the world will greatly exceed 450 ppm. "The world is going to have to create a carbon pie eventually and divide it up," says geochemist Wallace Broecker of Columbia University's Earth Institute. "A total amount of CO2 that each country will be able to add to the atmosphere."

And that's where the argument really starts. "For developed countries, when it comes to emissions space, their fundamental attitude is what is mine is mine. What I've taken away from you I've got to keep," says Yu Qingtai, ambassador from China. "For us, the developing countries, our position is our emissions space is under occupation and we want them back."

Of course, China emits the largest amount of greenhouse gases of any nation in the world, but on a per person basis it lags well behind the U.S., E.U. and Japan. "Every human being has an equal right to the resources of the atmosphere," says Dasgupta, arguing for a per person emissions limit. "How will this resource, which for the first time in history has become a resource which is limited in supply, how is to be divided in the global community?"

Of course, scientists such as Stanford University climatologist Stephen Schneider note that the impacts of climate change will fall most heavily on the poorest countries, which have the least capacity to adapt. Already, argues Quamrul Islam Chowdhury, the negotiator for Bangladesh, some 5 to 10 percent of his country will be washed away by climate change. "It will have also a huge impact not in terms of development but also in terms of human rights, migration and displacement."

"We are on the front lines of climate change," adds Crispin Gregoire, climate negotiator from Dominica. "Some of our islands will disappear. We accept that. We want an agreement that will address our survival."

And that means emissions cuts for everyone. "The question is not whether it is desirable to reduce the rate of growth of emissions in developing countries. Of course it is," Dasgupta says. "The question is who pays?"

The European Union, for its part, hopes that it's example might prove instructive with the launch of an emissions trading regime that has put a price of roughly 11 EU$ on every metric ton of carbon dioxide emissions. "The EU economy has been delinked effectively from the growth of further CO2 emisions," Falkenberg notes, pointing to economic growth of roughly 2.5 percent in 2007 paired with a decline in emissions of roughly 4 percent.

But it remains unclear how often words about commitments to combat climate change match real actions. "In international cooperation to fight climate change there is no shortage of legally binding documents. We have the Convention, we have the Kyoto Protocol," Yu says. "If you look at these basic documents there have been a list of commitments that have gone by without being met. So if we are going to have a positive outcome from Copenhagen it must make a difference when it comes to taking real action to match what we promise as sovereign nations."